According to the ATSG manual on the 722.3/4 transmissions "no postive contract in reserve gear" is either"

1. check lining plates and lip sealing rings on piston of LB3 and renew if required, or

2. renew one-way roller clutch in gear assembly.

It's most likely #1 - you blew a seal in the reverse piston. If so, there's nothing really wrong with your transmission. You could have another 100k in the forward operation. Replacing seals in the reverse piston is not that big a job - no need to even get into the inner workings of the transmission. The problem is of course you have to R and R the transmission which is six to seven hours. "Most people" recommend rebuilding the transmission once you've taken it out on a 14 year old car. But "most people" like your money. It isn't absolutely necessary if the transmission is otherwise operating properly. Sorta depends on what your future is with the car. But I agree, if transmission is OK better to rebuild yours than get a rebuilt in which you don't know the history.